The invention pertains to vibration sensing transducers, and particularly relates to geophones having a wide frequency range of operation along a primary axis and wherein damping means are utilized to suppress vibrations orthogonal to the primary axis of response.
Geophones and/or seismometers are widely used in the geophysical industry for measuring geophysical characteristics such as in oil exploration, and such devices are also widely used in military applications for sensing personnel or vehicular activity. Geophones are also utilized in underwater sound sensing electroacoustic transducers or hydrophones, as illustrated in the assignee's U.S. Pat. No. 3,720,909.
One of the common constructions for geophones includes the mounting of a mass, such as an annular wound coil, resiliently within the chamber of a casing and a permanent magnet affixed to the casing is located within the coil mass. The mass is mounted upon diaphragm springs having involute spring elements permitting relative movement between the coil mass and magnet along a primary axis of response. This relative axial movement as produced by vibrations imposed upon the casing produces electrical signals as the coil windings cut the lines of magnetic flux which are sensed and evaluated. Examples of this type of geophone or seismometer is shown in U.S. Pat. Nos. 3,239,804 and 3,451,040.
While the utilization of diaphragm springs having involute ribbon spring elements results in a concise spring with a low spring constant, and the sensing of vibrations along the primary response axis is sensitive, this type of diaphragm spring is subject to sensing and reacting to orthogonal or cross axis forces imposed upon the mass, and springs, which induce axial vibrations of the mass which are sensed and results in undesirable signals extraneous to those signals desired. In order to control unwanted vibrations within diaphragm springs, several damping or control methods have been proposed, such as coating the springs, as shown in U.S. Pat. No. 3,157,852, or utilizing extraneous spring engaging support elements as shown in U.S. Pat. No. 3,344,397. Neither of these approaches to the problem have proven satisfactory in all respects, particularly in geophones having a wide operating frequency range, such as through 8 octaves, as from 10 hertz to 2400 hertz. In many geophone applications the operating frequency range desired covers no more than 4 or 5 octaves, 10 hertz to 100 hertz, as an example, and in many geophone applications there is minimal interest in determining to any degree the exactness of the plane or planes in which the force motion occurs. The relatively simple geophone devices of limited frequency range are not usable over an 8 octave range, for instance, particularly when it is desired to accurately resolve relative vibrations in two or three orthogonal planes by the utilization of a plurality of geophones. Most geophone devices exhibit undesirable responses at certain frequencies, such as an output voltage greater then 5% to 10% of the voltage obtained when a geophone is excited along the axis of maximum response, when excited in a plane orthogonal to the axis of primary response.
In addition to the natural resonant frequency of the mass and springs in a direction parallel to the primary axis of vibration, the sensing of a second resonant frequency is inherent in geophone systems utilizing diaphragm springs. This frequency results from the mass of the coil and the springs bending in the plane coincident with their diameter, which, of course, is orthogonal to the geophone primary axis. This undesirable cross axis resonant frequency is higher than the natural resonant frequency along the primary axis. Further, diaphgram springs exhibit a third motional characteristic called "buckling". When motion is applied to the mass perpendicular to the primary axis, and parallel to the plane coincident with the diameter of the springs, the involute elements of the springs under compression tend to buckle. Due to imperfections in the spring, and its type of mounting in the geophone assembly, the buckling action results in translation of the mass along the primary axis producing an undesired response. The buckling mode resonance is characterized by its narrow band or high amplitude response, and it is also relatively high in frequency.
The aforementioned vibrations in the diaphragm springs resulting from orthogonal forces or vibrations imposed upon the mass produce geophone signals that adversely affect the desired geophone output, and with broad range frequency transducers wherein a plurality of geophones are related to each other as in U.S. Pat. No. 3,720,909, such undesired responses seriously affect the ability of the transducer assembly in which the geophone is utilized to accurately evaluate orthogonal vibrations. Orthogonal responses produce primary axis vibrations relating to the orthogonal forces in relation to the cosine of the angle of such forces, and unless the sensitivity to the orthogonal forces can be effectively damped, the output of the geophone assembly is adversely affected. Tests have indicated that the buckling mode of vibration produces the majority of the undesired response characteristics, and observation indicates that diaphragm springs commonly have more than one buckling mode.
While it is known to use damping fluids in vibration sensing devices, such as geophones and the like, in the conventional use of such damping fluids the void in which the vibrating elements are located is completely filled with the fluid. Typical arrangements of this type are illustrated in U.S. Pat Nos. 2,677,270 and 2,696,592. Experiments with geophones utilizing diaphragm springs wherein the involute spring elements are completely immersed in a damping fluid have not achieved significant improvement over the aforementioned problems resulting from orthogonal or cross axis vibrations. In that the involute spring elements of the diaphragm springs are equally immersed and exposed to the damping fluid, the resonant frequency of the springs may be modified by the fluid, but the resonant frequency is merely shifted, rather than being effectively suppressed and damped. This shifting of the resonant frequency in a wide range geophone assembly provides little improvement over the aforementioned problems.